It is conventional practice to bleach chemical pulps using bleaching agents such as chlorine and chlorine dioxide to produce pulps having the desired degree of whiteness. The use of chlorine has been described as detrimental to the environment particularly when contained in the effluent from the pulp mill, and thus efforts have been made to either eliminate chlorine and/or chlorine compounds or to reduce their use to an absolute minimum.
Peroxides such as hydrogen peroxide have been used in different stages of bleaching to obtain the desired brightness of pulp and reduce the amount of chlorine applied.
Ozone is also used as a bleaching agent to obtain the required brightness of chemical pulps. However, ozone has been known to degrade chemical pulps and thereby reduce their quality, in particular the strength characteristics of the chemical pulp. It is generally accepted in the industry that the viscosity of the pulp provides a very good indication of the strength potential of the pulp, i.e. the higher the viscosity the better the strength characteristics at a given kappa or permanganate no.
It has also been suggested to replace the water, which is the medium in which the pulp is normally contained, with a suitable organic solvent such as ethanol and methanol, and to bleach the pulp using ozone gas as described for example in Japanese patent 7849107 published May 4, 1978 by Ueshima. This patent describes a process for recovering methanol from the digestion of wood chips with sodium hydroxide and Na.sub.2 S and using this recovered methanol as a protector for the wood pulp during the ozone bleaching. In this patent air dried pulp was impregnated with methanol substantially free of water and was not acidified.
Japanese patent 7890403 published Aug. 9, 1978 to Ueshima et al. describes another application of methanol followed by ozone bleaching of the methanol containing pulp. Again, in this patent, the water free pulp was impregnated with methanol which was free of water and was not acidified. The impregnated pulp was subsequently ozonated. This patent did not show as good results as those obtained in the earlier patent described above.
An article entitled "The effect of cellulose protectors on ozone bleaching of kraft pulp" by Kamisima published in the Journal of Japanese Technical Association of the Pulp and Paper Industry, Vol. 31, 9, pp 62-70, September 1977, describes a number of different solvents that may be used to protect the pulp during an ozone bleaching stage. In these teachings, air dried pulp is treated with the organic solvents (alcohols), substantially free of water, and without addition of any acid, and then bleached with ozone. This publication indicates that ethanol is not effective in improving the viscosity of the ozone bleached pulp whereas the use of methanol does produce a positive result.
An article entitled "The use of ozone in bleaching of pulps" by Liebergott et al. 1991 Pulping Conference, TAPPI Proceedings, pp 1-23, provides a review of the literature on ozone bleaching and describes a number of different chemicals that have been tried, i.e. added to the pulp before and in combination with an ozone bleaching stage in attempts to overcome or significantly reduce the detrimental effects of the ozone stage on the quality of the bleached pulp. This article does not list dioxane as having been tried.
In Empire State Paper Research Institute report no. 54 titled "Ozone Bleaching of Kraft Pulps" by Rothenberg et al. October 1971 the use of various percentages of dioxane in water, combined with 1% acetic acid as a medium for ozone bleaching are reported. However the results obtained were not encouraging, and the ozone (lignin-carbohydrate) selectivity decreased when the concentration of dioxane increased above 35%.